(1) Field of the Invention
The present invention relates to sputtering methods and apparatus, and particularly to methods and apparatus for bias sputtering.
(2) Description of the Prior Art
Sputter coating is a well-known technique for coating a substrate with material eroded from the cathode of a low pressure gas electrical discharge (glow discharge) created between a cathode and an anode maintained at a suitable voltage difference in a low pressure gas atmosphere. A glow discharge contains an abundance of positively charged ions formed by collision of electrons with atoms of the low pressure gas. The resulting ions are attracted to the negatively charged cathode, which they impact with considerable energy. This ion impact dislodges cathode surface atoms which will then condense on, and thereby coat, the surface of any object placed near the cathode.
Since sputtering is a low pressure process, it must be carried out in a hermetically sealed chamber, which is first evacuated and then back-filled with a suitable sputtering gas, usually argon, and maintained at the proper sputtering pressure, typically 5 to 40 millitorr.
In many coating applications the substrate to be coated is placed on the anode of the gas discharge, since the anode is usually directly opposite the cathode, in a suitable location for coating by dislodged cathode atoms. Most sputtering systems use an anode at ground potential and apply a large negative voltage to the cathode; the grounded sputtering chamber then becomes an auxiliary anode.
Bias sputtering is a modified sputter coating technique in which a bias potential, usually negative, is applied to the substrate which is to be coated. This bias potential causes some of the gas discharge ions to be attracted to the substrate during the deposition process. The ion impact can produce desirable changes in the nature of the sputter coating. An important use of the bias sputtering technique is in the so-called reactive sputtering process. During active sputtering, a chemically active gas, such as oxygen or nitrogen, is added to or substituted for the usual inert sputtering gas (e.g., argon). Reactive species of such active gas are created in the glow discharge region, along with the usual argon ions, and these species react with sputtered target atoms deposited on the substrate to form a desired compound. The reactive sputtering technique thus permits sputtering from a pure metal target, aluminum for example, to produce a compound coating on the substrate (e.g., aluminum oxide or aluminum nitride). Reactive sputtering has economic advantages because the sputtering rate from a metal target is much higher than from a target composed of the metallic compound.
Bias sputtering, by placing a negative potential on the substrate, increases the chemical reaction rate by, among other things, attracting the positively charged reactive gas species or ions. Substrate biasing has a limitation, however, because ion bombardment can also cause undesirable substrate heating and gas ion implantation in the coating. Thus, the problem is to obtain a large flux of low-energy ions (energy levels of 20 to 100 electron-volts) which are sufficient for the chemical reaction process at the substrate surface, without getting a significant amount of high-energy ion bombardment.
More complete descriptions of prior art sputter techniques, including reactive sputtering and bias sputtering, may be found in "Gas Discharge Processes", by B. M. Chapman (J. Wiley & Sons, 1980) and "Thin Film Processes", edited by J. L. Vossen and W. Kern (Academic Press, Inc., 1978). One way known to the prior art for increasing ionization in a gas discharge is by use of magnetic field traps. Such magnetically enhanced sputtering was first developed by Penning, as disclosed in U.S. Pat. No. 2,146,025. Variations of the Penning structures have been used to achieve efficient high-rate sputtering cathodes (U.S. Pat. Nos. 3,282,815; 3,282,816; 3,325,394; 3,369,991; 3,711,398; 3,878,085; 3,995,187; 4,030,996; 4,031,424; 4,041,353; 4,111,782; 4,116,793; 4,166,018; 4,194,062; 4,198,283). In addition, the above-cited Vossen text shows at page 32 a radio-frequency (rf) powered bias sputtering apparatus having both cathode and anode magnets for generating a magnetic field between the cathode and the substrate supporting bias electrode. These magnets are incapable of creating a re-entrant field magnetic electron trap enclosing the substrate support surface, however. Such closed-loop fields have been use only in connection with high-rate sputtering sources (i.e., cathodes).